1. Field of the Invention
The present invention relates to fabrication of a display device, and particularly, to a method of fabricating a liquid crystal display device.
2. Description of the Related Art
Cathode ray tube (CRT) monitors have been commonly used for displaying information in television and computer systems. The CRT monitors produce high quality images and have relatively high brightness. However, as sizes of image display screens increase, depths of the CRT monitors have increased, thereby occupying a very large volume. In addition, weight of the CRT monitors have been problematic for use in portable display devices. In order to address these problems, flat panel display devices, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, organic electro-luminescence display (OELD) devices, light emitting diode (LED) devices, and field emission display (FED) devices, have been substituted for the CRT monitors. Among these various flat panel display devices, the liquid crystal display (LCD) devices are commonly used in notebook and desktop computers because of their low power consumption.
FIG. 1 is a cross sectional view of liquid crystal display (LCD) device according to the related art. In FIG. 1, an LCD device includes a lower substrate 10, an upper substrate 20, and a liquid crystal material layer 15 formed in between the upper and lower substrates 10 and 20.
In addition, a thin film transistor T and a pixel electrode 7 are formed on the lower substrate 10, wherein the thin film transistor T includes a gate electrode 1 to which a scan signal is supplied, a semiconductor layer 3 for transmitting a data signal corresponding to the scan signal, a gate insulating layer 2 for electrically isolating the semiconductor layer 3 and the gate electrode 1, a source electrode 4 formed on an upper part of the semiconductor layer 3 for supplying the data signal, and a drain electrode 5 for supplying the data signal to the pixel electrode 7. The semiconductor layer 3 comprises an active layer 3a formed by depositing amorphous silicon (a—Si) and an n+ doped ohmic contact layer 3b on both upper sides of the active layer 3a. A passivation layer 6 and the pixel electrode 7 are formed on the thin film transistor T, and a first alignment layer 4a is formed on an upper part of the pixel electrode 7 for aligning liquid crystal molecules of the liquid crystal material layer 15. The pixel electrode 7 is made of a transparent conductor, such as indium tin oxide (ITO) or indium zinc oxide (IZO), so that light can be transmitted through the pixel electrode 7.
In FIG. 1, a black matrix 12 is formed on the upper substrate 20 for preventing the light from leaking between adjacent pixels, and color filters 11 of red (R), green (G), and blue (B) are formed on the black matrix 12 in order to produce colored light. In addition, a flattening layer (not shown) can be formed on the color filter 11 for flattening the color filters 11 and for improving adhesive bonding to a common electrode 13 that is subsequently formed on the color filters 11. A second alignment layer 4b is formed on the common electrode 13 for aligning the liquid crystal molecules of the liquid crystal material layer 15. A transparent conductor, such as ITO or IZO, is used as the common electrode 13 so that the light can be transmitted through the common electrode 13.
To fabricate the LCD device, several thin film deposition and photolithographic processes should be performed. For example, to fabricate the thin film transistor T, the color filters 11, and the black matrix 12, a photoresist pattern is formed by applying photoresist material. Then, the photoresist material undergoes exposure and strip processes using a mask, and an etching process is performed using the photoresist pattern as a mask. Accordingly, the process for forming the photoresist material includes rather complex fabrication processes and is not suitable for large area display devices. Thus, a printing method is used to pattern the photoresist material without the need for the exposure process.
FIGS. 2A to 2C are cross sectional views of a printing process according to the related art. In FIG. 2A, a cliché 24 having concave grooves 23 formed at a position corresponding to a desired pattern to be formed on a substrate is prepared, wherein a resist material 31 is deposited. Then, a doctor blade 32 is moved across a surface of the cliché 24 to deposit the resist material 31 into the concave grooves 23.
In FIG. 2B, the resist material 31 filled within the concave grooves 23 of the cliché 24 is transferred as a plurality of resist portions 31 onto a surface of a printing roll 33 as the printing roll 33 contacts the surface of the cliché 24. The printing roll 33 is formed to have the same width as a substrate 30 (in FIG. 2C) onto which the resist portions 31 subsequently will be applied. In addition, a circumference of the printing roll 33 is formed to have the same length of the substrate 30 (in FIG. 2C).
In FIG. 2C, the resist portions 31 are transferred from the printing roll 33 onto a surface of the substrate 30 as the printing roll 33 is rotated. Although not shown, the substrate 30 may include an etching object layer, wherein the resist portions 31 actually contact the etching object layer. Then, the resist portions 31 are irradiated with ultraviolet (UV) light, or the resist portions 31 are dried using heat to form a resist pattern 31. Accordingly, since a depth of the resist portions 31 formed on the cliché 24 are all the same, the resist pattern 31 may have various defects that only correspond to a width of the resist pattern 31
For example, when spacing between adjacent resist portions 31 is relatively narrow, the resist pattern 31 may include openings, as shown in FIG. 3, wherein a hole 40 may be generated at an end of the resist pattern 31, or a thickness of a center portion of the resist pattern 31 may become thinner than a thickness of an edge portion of the resist pattern 31. Accordingly, the spacings between the adjacent resist portions 31 and the thicknesses of the center and edge portions of the resist pattern 31 may vary due to differences in pressure between the printing roll 33 and the substrate 30 when the resist portions 31 are transferred from the cliché 24 onto the substrate 30. Thus, the defects of the resist pattern 31 cause electrical short circuits or electrical open circuits between the etching object layer (not shown), which is subsequently etched using the resist pattern 31 as a mask, thereby deteriorating image quality.